Casting products are made in molds by first preparing a mold which might be a metal permanent mold or a sand mold. Sand molds are destroyed when each casting is poured or otherwise applied thereto whereas permanent molds can be used to make many castings.
Permanent molds are used to make many parts of a single design. Permanent molds are commonly used to produce iron pipe, engine blocks, engine heads, cylinder liners and other metal products. These manufactured products must have a geometrical shape so that a mold can be machined out of steel, copper or other well known metal alloys employed for constructing molds in the industry.
In the casting process a cavity is first shaped in a mold. The mold inner cavity surface is the active surface and has a configuration corresponding to the exterior surface of the product desired to be manufactured. Molten metal is poured into the mold cavity and takes the shape of the cavity. As the metal cools it solidifies into a metal casting product having a shape corresponding to the mold cavity is produced.
Steel permanent molds are used to make millions of tons of pipe each year. The permanent molds are commonly made from steel alloys that may cost a pipe manufacturer upwards of $250,000.00 USD to purchase. The permanent mold for pipes may be used hundreds or thousands of times before thermal cracks develop on the cavity internal active surface due to the exposure of the active surface to the high temperature molten metal. The expensive mold may begin to leak and then it must be scrapped and taken out of service costing a business lost productivity. In the prior art insulating coatings have been applied to the active mold surface forming a thermal barrier that inhibits deterioration of the mold due to the overheating by the molten metal.
The insulating coatings slow down heat transfer to the mold. Thermal stress fractures will occur on the permanent metal mold with the life being shortened by higher heat transfer rates. The insulating coating reduces the heat transfer rate and extends the life of the mold by reducing the rate of thermal stress fracture formation.
The prior art insulating coatings are also useful as a coating for sand molds and cores to prevent the aluminum, bronze, gray iron, ductile iron, and steel from adhering to the sand mold or core.
The prior art employs an insulating coating to protect centrifugal casting molds used to produce centrifugally cast products. There are many prior art insulating coatings used on permanent molds, sand molds and sand cores. Commonly used coating refractory material in the prior art includes silica, mullite, andalusite, kaolinite, zircon, zirconia, diatomaceous earth, western bentonite, southern bentonite, specially treated high expansion bentonites, graphite, alumina and silica-alumina compounds. These are crystalline materials.
The prior art insulating coatings often include well known suspension agents to assist in maintaining the refractory materials in a uniformly distributed suspension during extended periods of storage prior to use. Further, many of the prior art insulating coatings include biocides to prevent fungal and plant growth during storage.
In prior art, stationary non-centrifugal molds, a slurry of a finely divided particulate refractory material, such as zircon powder or silica powder, has been used as an insulating coating, see U.S. Pat. No. 1,662,354 to Harry M. Williams, and employed with centrifugal casting, see U.S. Pat. No. 3,527,285 to Fred J. Webbere. Also in rotary casting of iron pipe as seen in U.S. Pat. No. 4,058,153 (Pierrel), a coating is used for protection of centrifugal casting molds for cast iron pipes. The insulating coating comprises of powder refractory materials such as silica and bentonites in a suspension.
Prior art insulating coating materials as described above are often dispersed in water suspension and sprayed onto the mold. A drawback of a water suspension coating is that considerable water vapor and maybe some hydrogen are generated in a flash when the high temperature molten metal is poured into contact with the mold coating. The water vapor and gases generate bubbles which blow into the casting forming gas holes and otherwise compromising the strength and integrity of the cast product. In the casting industry these holes are often called pin-holes, leaks, blows and gas-holes.
Similarly gas from organic suspension agents, inorganic suspension agents or wetting agents employed in some prior art insulating coatings produce hydrogen, The hydrogen expands and blows out of the insulating coatings into the casting product forming pin-holes and resulting in a loss of pressure tightness therein. Such defects can either cause pipe castings to leak upon solidification, or the pinholes may just result in a shorter life expectancy for the mold. Accordingly the defective pipe castings must be scrapped resulting in higher average manufacturing costs per unit of good product because of the lost labor, core materials, overhead and lost production time.
Another prior art method of providing a protective insulating thermal barrier on molds is a technique referred to as “dry spray”. As disclosed in U.S. Pat. No. 1,949,433 to Norman F. S. Russell et al, such methods employ a carrier gas to move the particulate refractory material onto the active mold surface immediately in advance of the metal being cast. This coating method depends upon centrifugal force to establish and maintain a coating layer of the refractory material on the mold. Since no water is used in this coating technique fewer pinhole problems arise on account of lower gas content in the coating. Pinholes and leaks still occur however in iron castings manufactured with this dry spray process because the metal turbulence during spraying allows carbon monoxide and carbon dioxide induced pinholes to form. Also, the “dry spray” is difficult to uniformly distribute and maintain over the surface of the mold when the metal is sprayed.
Still other prior art insulating coatings have employed resins and organic adhesives for bonding refractory materials onto the active mold surface. The resins and organic adhesives upon being heated by the molten metal become chemically reactive forming blowout gases, such as hydrogen, carbon monoxide, and carbon dioxide. The only way to vent the volume of these gases out of the mold is to drill hundreds or thousands of holes into the back side of the mold.
In addition, these blowout gases from said resins and organic adhesives such as carbon monoxide are toxic and environmentally objectionable. As environmental concerns in the United States increase, the release of potentially harmful gases are likely to become more and more regulated. Similarly, refractory materials for insulating coatings in the past have been suspended in alcohol and other organic solvents. There is a preference to limit the use of organic solvents because of their effects on the atmosphere, their flammability and explosion potential.
When prior art water-based insulating coatings are applied to the surface of a mold cavity most of the carrier water evaporates on account of the elevated temperature of molds during casting. In centrifugal pipe casting the temperature of the mold at its active cavity surface is typically in the range of 400° F.-600° F. In prior art casting molds, a not insignificant amount of water remains in some existing pores of the prior art coatings. In addition a not insignificant amount of water is chemically bound to certain types of compounds in the prior art insulating coatings. This not insignificant amount of water remains in the insulating coating even after an extended period of time and remains until the coating temperature becomes much higher, a temperature somewhere above 1,200° F. In many metal casting processes the temperature of the molten metal is higher than 1,200° F., for instance molten iron alloy temperatures are in the range of 2,300° F.-2,500° F. This remaining water vaporizes upon contact with the molten metal to form blowout gases. Pinholes and/or other defects arise in the castings formed by prior art molds on account of these blowout gases.